Method for producing perovskite-type composite oxide

ABSTRACT

To provide a highly safe and hygienic method for industrially efficiently producing a perovskite-type composite oxide at low temperatures in heat treatment, in which the resulting perovskite-type composite oxide can maintain the catalytic activity of a noble metal at a high level over a long time, in a method for producing a perovskite-type composite oxide, a perovskite-type composite oxide is produced by mixing organometal salts of all elementary components constituting the perovskite-type composite oxide to prepare a precursor of the perovskite-type composite oxide, or mixing one or more organometal salts of part of the elementary components constituting the perovskite-type composite oxide with the other elementary components prepared as alkoxides of the respective elements, a coprecipitate of salts of the respective elements or a citrate complex of the respective elements to prepare a precursor of the perovskite-type composite oxide, and heat-treating the precursor of the perovskite-type composite oxide.

TECHNICAL FIELD

The present invention relates to a method for producing aperovskite-type composite oxide. More specifically, it relates to aperovskite-type composite oxide which is advantageously used as anexhaust gas purifying catalyst for efficiently purifying carbon monoxide(CO), hydrocarbons (HC), and nitrogen oxides (NOx).

BACKGROUND ART

Noble metals such as Pt (platinum), Rh (rhodium) and Pd (palladium)supported by perovskite-type composite oxides represented by the generalformula: ABO₃ have been widely known as three-way catalysts which cansimultaneously clean up carbon monoxide (CO), hydrocarbons (HC), andnitrogen oxides (NOx) contained in emissions.

When such a noble metal is merely supported by the perovskite-typecomposite oxide, however, the noble metal undergoes grain growth at thesurface of the perovskite-type composite oxide in use at hightemperatures. To avoid this, techniques of incorporating a noble metalinto a perovskite-type composite oxide as its constitutional elementhave been proposed.

Japanese Laid-open (Unexamined) Patent Publication No. Hei 6-100319, forexample, discloses a method of producing a perovskite-type compositeoxide. In this method, an

aqueous solution containing citric acid and salts of elementsconstituting the perovskite-type composite oxide and including the noblemetals is initially prepared, and the aqueous solution is dried toobtain a complex between citric acid and the respective elements. Thecomplex is thermally decomposed by heating at 350° C. or higher invacuum or in an atmosphere of inert gas to obtain a precursor. Theprecursor is subjected to a heat treatment under an oxidative atmosphereto obtain the perovskite-type composite oxide.

Japanese Laid-open (Unexamined) Patent Publication No. Hei 8-217461discloses a method for producing a perovskite-type composite oxide. Inthis method, a solution of alkoxides of elements constituting theperovskite-type composite oxide other than the noble metal is initiallyprepared. The alkoxide solution is hydrolyzed by adding an aqueoussolution of a salt of the noble metal, and the solvent and fluid areremoved to obtain a precursor. The resulting precursor is subjected to aheat treatment at 500° C. to 500° C. under an oxidative atmosphere toobtain the perovskite-type composite oxide.

Japanese Laid-open (Unexamined) Patent Publication No. 2000-15097discloses a method for producing a perovskite-type composite oxide. Inthis method, a salt of the noble metal constituting the perovskite-typecomposite oxide is mixed with an organic polymer to obtain a colloidsolution of the noble metal. The colloid solution is mixed withalkoxides of elements constituting the perovskite-type composite oxideand is then hydrolyzed to obtain a precursor. The precursor is dried andis subjected to a heat treatment to obtain the perovskite-type compositeoxide.

In these perovskite-type composite oxides, the noble metals can befinely and highly dispersed therein and can maintain their highcatalytic activities even in long-term use. This is because of aself-regenerative function, in which the noble metals undergo repetitivesolid-solution under an oxidative atmosphere and deposition under areducing atmosphere. These findings have been obtained in recent years.

Any of the above-mentioned methods, however, has limitations toefficiently disperse the noble metal in the perovskite-type compositeoxide for a higher rate of solid-solution and to improve the catalyticactivity.

The above-mentioned methods use the noble metal as an aqueous solutionof a salt thereof such as nitrate, chloride, or dinitrodiammine salt andmay invite an abrupt exothermic reaction in the heat treatment, whichmay in turn invite bubbling over of the resulting powder. To avoid this,the temperature must be gradually raised, which constitutes asignificant limitation in industrial production. In addition, the heattreatment yields harmful by-products such as nitric acid, hydrochloricacid, or amines but these must be avoided for safety or hygiene.

In addition, the above-mentioned methods require considerably hightemperatures in the heat treatment to form a single phase of theperovskite-type composite oxide. The heat treatment at such hightemperatures, however, inevitably invites a decreased specific surfacearea.

DISCLOSURE OF INVENTION

Accordingly, an object of the present invention is to provide a highlysafe and hygienic method for industrially efficiently producing aperovskite-type composite oxide at low temperatures in a heat treatment,in which the resulting perovskite-type composite oxide can maintain thecatalytic activity of a noble metal at a high level over a long time.

The present invention therefore provides a method for producing aperovskite-type composite oxide, which comprises the steps of preparinga precursor of the provskite-type composite oxide by mixing at leastorganometal salts of elementary components constituting theperovskite-type composite oxide, and heat-treating the precursor of theperovskite-type composite oxide.

In the preparation step, the precursor of the perovskite-type compositeoxide can be prepared by mixing one or more organometal salts of part ofthe elementary components constituting the perovskite-type compositeoxide with the other elementary components.

The other elementary components can be prepared as alkoxides of therespective elements.

The other elementary components can also be prepared as a coprecipitateof salts of the respective elements or a citrate complex of therespective elements.

The part of the elementary components is preferably one or more noblemetals.

The organometal salts of the elementary components are preferablyorganic carboxylic acid salts of the elementary components and/or ametal complex of the elementary components including at least oneselected from the group consisting of β-diketone compounds, β-ketoestercompounds, and β-dicarboxylic ester compounds.

The perovskite-type composite oxide in the present invention ispreferably a perovskite-type composite oxide represented by thefollowing general formula (1):ABMO₃  (1)wherein A represents at least one element selected from rare-earthelements, alkaline earth metals, and Ag; B represents at least oneelement selected from Al and transition metals excluding platinum groupelements and rare-earth elements; and M represents one or more platinumgroup elements.

The method for producing a perovskite-type composite oxide of thepresent invention is free from an abrupt exothermic reaction in the heattreatment, is thereby substantially free from the bubbling over of theresulting powder and can industrially efficiently carry out the heattreatment. In addition, the heat treatment yields organic substancesfrom decomposed organometal salts but does not yield harmfulby-products, thus improving the safety and hygiene.

Further, the method for producing a perovskite-type composite oxide ofthe present invention can yield a perovskite-type composite oxide of asingle phase even at relatively low temperatures and can thereby preventthe specific surface area of the resulting perovskite-type compositeoxide from decreasing.

The perovskite-type composite oxide produced by the method for producinga perovskite-type composite oxide of the present invention allows thenoble metal to be efficiently dispersed therein thereby to increase arate of solid-solution. In the perovskite-type composite oxide, thenoble metal is finely and highly dispersed and can maintain its highcatalytic activity even in long-term use. This is because of theself-regenerative function in which the noble metal repetitivelyundergoes solid-solution under an oxidative atmosphere and depositionunder a reducing atmosphere.

BEST MODE FOR CARRYING OUT THE INVENTION

The method for producing a perovskite-type composite oxide of thepresent invention can be used in, but not specifically limited to, theproduction of a perovskite-type composite oxide represented by thefollowing general formula (1):ABMO₃  (1)wherein A represents at least one element selected from rare-earthelements, alkaline earth metals, and Ag; B represents at least oneelement selected from Al and transition metals excluding platinum groupelements and the rare-earth elements; and M represents one or moreplatinum group elements.

More specifically, this composite oxide has a perovskite structure andcomprises at least one element selected from rare-earth elements,alkaline earth metals, and Ag on the A site, and one or more platinumgroup elements and at least one element selected from Al and transitionmetals excluding platinum group elements and rare-earth elements on theB site.

Examples of the rare-earth elements represented by A include rare-earthelements each having a valence of 3 as the only valence, such as Sc(scandium), Y (yttrium), La (lanthanum), Nd (neodymium), Pm(promethium), Gd (gadolinium), Dy (dysprosium), Ho (holmium), Er(erbium), and Lu (lutetium); rare-earth elements each having a variablevalence of 3 or 4, such as Ce (cerium), Tb (terbium), and Pr(praseodymium); and rare-earth elements each having a variable valenceof 2 or 3, such as Eu (europium), Tm (thulium), Yb (ytterbium), and Sm(samarium).

These rare-earth elements can be used alone or in combination. Therare-earth element having a valence of 3 as the only valence ispreferably used, optionally in combination with the rare-earth elementhaving a variable valence of 3 or 4. More preferably, La, Nd, and/or Yis used optionally in combination with Ce and/or Pr.

Examples of the alkaline earth metals represented by A include Be(beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium),and Ra (radium). These alkaline earth metals can be used alone or incombination. Among them, Mg, Ca, Sr, and Ba are preferred.

The at least one element selected from the rare-earth elements, thealkaline earth metals, and Ag to be contained on the A site is notspecifically limited. Preferably, at least one element selected from La,Nd, and Y is essentially contained and at least one element selectedfrom Ce, Pr, Mg, Ca, Sr, Ba, and Ag is optionally contained on the Asite. In this case, the atomic ratio x of at least one element selectedfrom Ce, Pr, Mg, Ca, Sr, Ba, and Ag preferably satisfies the followingrelation: 0≦x≦0.5. Namely, the atomic ratio (1−x) of at least oneelement selected from La, Nd, and Y preferably satisfies the followingrelation: 0.5≦(1−x)≦1.0.

The transition metals represented by B excluding the platinum groupelements and the rare-earth elements are not specifically limited andinclude elements having atomic numbers of 22 (Ti) through 30 (Zn),atomic numbers of 40 (Zr) through 48 (Cd), and atomic numbers of 72 (Hf)through 80 (Hg) in the Periodic Table of Elements (IUPAC, 1990), exceptfor Pd and Co. Specific examples thereof include, but are not limitedto, Cr (chromium), Mn (manganese), Fe (iron), Co (cobalt), Ni (nickel),and Cu (copper). These transition elements can be used alone or incombination.

Examples of the platinum group elements represented by M include Ru(ruthenium), Rh (rhodium), Pd (palladium), Os (osmium), Ir (iridium),and Pt (platinum). These platinum group elements can be used alone or incombination. Among them, Ru, Rh, Pd, Ir, and Pt are preferred.

On the B site, at least one element selected from the platinum groupelements and at least one element selected from Al and transition metalsexcluding the platinum group elements and the rare-earth elements areessentially contained. The atomic ratio y of the platinum group elementson the B site preferably satisfies the following relation: 0<y≦0.5.Namely, the atomic ratio (1−y) of Al and the transition metals excludingthe platinum group elements and the rare-earth elements preferablysatisfies the following relation: 0.5≦(1−y)<1.0.

As is described above, the method for producing a perovskite-typecomposite oxide of the present invention is more preferably used for theproduction of a perovskite-type composite oxide represented by thefollowing general formula (2):A_(1−x)A′_(x)B_(1−y)B′_(y)O₃  (2)wherein A represents at least one element selected from Y, La, and Nd;A′ represents at least one element selected from Ce, Pr, Mg, Ca, Sr, Ba,and Ag; B represents at least one element selected from Cr, Mn, Fe, Co,Ni, Cu, and Al; B′ represents at least one element selected from Ru, Rh,Pd, Ir, and Pt; x represents an atomic ratio satisfying the followingrelation: 0≦x≦0.5; and y represents an atomic ratio satisfying thefollowing relation: 0<y≦0.5.

According to the method for producing a perovskite-type composite oxideof the present invention, at least organometal salts of elementarycomponents constituting the perovskite-type composite oxide are mixed toprepare a precursor of the perovskite-type composite oxide in thepreparation step.

In this preparation step, the precursor may be prepared by mixingorganometal salts of all the elementary components constituting theperovskite-type composite oxide or by mixing one or more organometalsalts of part of the elementary components constituting theperovskite-type composite oxide with the other elementary components.

Examples of the organometal salts of the elementary componentsconstituting the perovskite-type composite oxide include carboxylic acidsalts of the elementary components derived from, for example, acetatesor propionates; and metal chelate complexes of the elementary componentsderived from, for example, β-diketone compounds or β-ketoester compoundsrepresented by the following general formula (3) and/or β-dicarboxylicester compounds represented by the following general formula (4):R¹COCHR³COR²  (3)wherein R¹ represents an alkyl group having 1 to 6 carbon atoms, afluoroalkyl group having 1 to 6 carbon atoms or an aryl group; R²represents an alkyl group having 1 to 6 carbon atoms, a fluoroalkylgroup having 1 to 6 carbon atoms, an aryl group or an alkyloxy grouphaving 1 to 4 carbon atoms; and R³ represents a hydrogen atom or analkyl group having 1 to 4 carbon atoms,R⁵CH(COOR⁴)₂  (4)wherein R⁴ represents an alkyl group having 1 to 6 carbon atoms; and R⁵represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.

In above-mentioned general formulas (3) and (4), examples of the alkylgroups each having 1 to 6 carbon atoms as R¹, R², and R⁴ include methyl,ethyl, propyl, isopropyl, n-butyl, s-butyl, t-butyl, t-amyl, andt-hexyl. Examples of the alkyl groups each having 1 to 4 carbon atoms asR³ and R⁵ include methyl, ethyl, propyl, isopropyl, n-butyl, s-butyl,and t-butyl. The fluoroalkyl groups each having 1 to 6 carbon atoms asR¹ and R² include, for example, trifluoromethyl. The aryl groups as R¹and R² include, for example, phenyl. Examples of the alkyloxy grouphaving 1 to 4 carbon atoms as R² include methoxy, ethoxy, propoxy,isopropoxy, n-butoxy, s-butoxy, and t-butoxy.

More specific examples of the β-diketone compounds include2,4-pentanedione, 2,4-hexanedione, 2,2-dimethyl-3,5-hexanedione,1-phenyl-1,3-butanedione, 1-trifluoromethyl-1,3-butanedione,hexafluoroacetylacetone, 1,3-diphenyl-1,3-propanedione, anddipivaloylmethane. Examples of the β-ketoester compounds include methylacetoacetate, ethyl acetoacetate, and t-butyl acetoacetate. Examples ofthe β-dicarboxylic ester compounds include dimethyl malonate and diethylmalonate.

The organometal salts of the elementary components can be prepared as anorganometal salt solution by mixing organometal salts of the respectiveelements of the elementary components. The organometal salt solutioncontaining the organometal salts of the respective elements of theelementary components can be prepared, for example, by adding theorganometal salts of the respective elements of the elementarycomponents to an organic solvent so as to establish a predeterminedstoichiometric ratio in the perovskite-type composite oxide and mixingthem with stirring. The organic solvent is not specifically limited, aslong as it can dissolve the organometal salts of the respective elementsof the elementary components, and includes, for example, aromatichydrocarbons, aliphatic hydrocarbons, alcohols, ketones and esters.Among them, a ketone such as acetone and/or an aromatic hydrocarbon suchas benzene, toluene, or xylene is preferably used.

When all elementary components constituting the perovskite-typecomposite oxide are prepared as the organometal salt solution, theprecursor of the perovskite-type composite oxide can be prepared, forexample, by distilling off the solvent and thermally decomposing theorganometal salts of the respective elements of the elementarycomponents by gradually heating the residue to about 400° C.

When the precursor is prepared by mixing one or more organometal saltsof part of the elementary components constituting the perovskite-typecomposite oxide with the other elementary components, the precursor canbe prepared by preparing an organometal salt solution of the organometalsalts of the part of the elementary components in the above-mentionedmanner, separately preparing, for example, alkoxides of the respectiveelements of the other elementary components, a coprecipitate of salts ofthe respective elements, or a citrate complex of the respectiveelements, and mixing the former with the latter.

The part of the elementary components used as organometal salts isfreely selected from among the respective elements constituting theperovskite-type composite oxide, of which preferred examples are theplatinum group elements such as Ru, Rh, Pd, Os, Ir, and Pt and the noblemetals such as Ag. These elementary components used as organometal saltscan be used alone or in combination.

The organometal salt solution containing the part of the elementarycomponents can be prepared, for example, by adding the organometal saltsof the respective elements of the elementary components to an organicsolvent so as to establish a predetermined stoichiometric ratio in theperovskite-type composite oxide and stirring the resulting mixture, asis described above. Any of the above-mentioned organic solvents may beused as the organic solvent herein.

The other elementary components are not specifically limited and are theremainder of the part of the respective elements constituting theperovskite-type composite oxide used as the organometal salts. They canbe prepared as, but not limited to, alkoxides of the respective elementsof the other elementary components, a coprecipitate of salts of therespective elements, or a citrate complex of the respective elements, asdescribed above.

Examples of the alkoxides of the other elementary components includealcholates derived from the respective elements of the other elementarycomponents and alkoxy such as methoxy, ethoxy, propoxy, isopropoxy, andbutoxy; and alkoxyalcholates of the respective elements represented bythe following general formula (5):E[OCH(R⁵)—(CH₂)_(a)—OR⁶]s  (5)wherein E represents the respective element; R⁵ represents a hydrogenatom or an alkyl group having 1 to 4 carbon atoms; R⁶ represents analkyl group having 1 to 4 carbon atoms; a is an integer of 1 to 3; and sis an integer of 2 or 3.

More specific examples of the alkoxyalcholates are methoxyethylate,methoxypropylate, methoxybutylate, ethoxyethylate, ethoxypropylate,propoxyethylate, and butoxyethylate.

The alkoxides of the other elementary components can be prepared, forexample, as a alkoxide mixed solution by adding the alkoxides of therespective elements of the other elementary components to an organicsolvent so as to establish a predetermined stoichiometric ratio in theperovskite-type composite oxide and mixing them with stirring. Any ofthe above-listed organic solvents can be used as the organic solventherein.

When the other elementary components are prepared as such a alkoxidemixed solution, the precursor of the perovskite-type composite oxide canbe prepared by mixing the organometal salt solution of the part of theelementary components with the alkoxide mixed solution to prepare ahomogenous mixed solution, coprecipitating the homogenous mixed solutionupon hydrolysis by adding water thereto, and drying the resultingprecipitate typically by vacuum drying or forced-air drying.

The coprecipitate of the other elementary components can be prepared,for example, by preparing an aqueous mixed salt solution containingsalts of the respective elements of the other elementary components in apredetermined stoichiometric ratio in the perovskite-type compositeoxide and allowing the aqueous mixed salt solution to coprecipitate byadding a neutralizing agent thereto.

Examples of the salts of the respective elements include inorganic saltssuch as sulfates, nitrates, chlorides, and phosphates; and salts oforganic acids, such as acetates and oxalates, of which nitrates and/oracetates are preferred. The aqueous mixed salt solution can be prepared,for example, by adding salts of the respective elements of the otherelementary components to water in such proportions as to establish apredetermined stoichiometric ratio in the perovskite-type compositeoxide and mixing them with stirring.

Then, the aqueous mixed salt solution is coprecipitated by adding theneutralizing agent thereto. The neutralizing agent includes, but is notspecifically limited to, ammonia; organic bases such as triethylamineand pyridine; and inorganic bases such as sodium hydroxide, potassiumhydroxide, potassium carbonate, and ammonium carbonate. The neutralizingagent is added dropwise to the aqueous mixed salt solution so that thesolution after the addition of the neutralizing agent has a pH of about6 to 10. This dropwise addition efficiently coprecipitates the salts ofthe elements.

When the other elementary components are formed into such acoprecipitate, the resulting coprecipitate is separated typically byfiltration or centrifugation and is fully washed with water to removeby-produced salts. The coprecipitate from which salts have been removedis dried and, where necessary, mixed with water to obtain a pasteslurry, is mixed with the organometal salt solution of the part ofelementary components with stirring to obtain a homogenous slurry. Theslurry is then evaporated to dryness as intact or after filtration ifnecessary. The precursor of the perovskite-type composite oxide is thusprepared.

The citrate complex of the other elementary components can be prepared,for example, as an aqueous citrate mixed salt solution containing citricacid and salts of the respective elements of the other elementarycomponents in such proportions so as to establish a predeterminedstoichiometric ratio in the perovskite-type composite oxide.

The salts of the respective elements of the other elementary componentsmay be the same as those listed above, of which acetates and/or nitratesare preferably used. The aqueous citrate mixed salt solution can beprepared, for example, by preparing the aqueous mixed salt solution bythe above-mentioned procedure and mixing the resulting aqueous mixedsalt solution with an aqueous solution of citric acid. The amount ofcitric acid is, for example, preferably about 2 to 3 moles per 1 mole ofthe perovskite-type composite oxide.

When the other elementary components are formed into such an aqueouscitrate mixed salt solution, the organometal salt solution of the partof the elementary components is added to the aqueous citrate mixed saltsolution and is mixed with stirring to obtain a homogenous slurry, fluidof the slurry is evaporated to dryness by heating under reduced pressureto obtain a mixture of organometal salts of the part of the elementarycomponents and the citrate complex of the other elementary components.The organometal salts of the part of the elementary components and thecitrate complex of the other elementary components are then thermallydecomposed by heating the mixture gradually to about 400° C. This yieldsthe precursor of the perovskite-type composite oxide.

When the other elementary components are formed into the aqueous citratemixed salt solution, the precursor of the perovskite-type compositeoxide can also be prepared in the following manner. The water in theaqueous citrate mixed salt solution is evaporated to dryness by heatingunder reduced pressure to obtain a citrate complex of the otherelementary components, and the citrate complex is thermally decomposedto obtain a decomposed product. The organometal salt solution of thepart of the elementary components is added to and they are mixed withstirring to obtain a homogeneous slurry, and the solvent in thehomogeneous slurry is evaporated to dryness.

According to the method for producing a perovskite-type composite oxideof the present invention, the above-prepared precursor of theperovskite-type composite oxide is heat-treated in the heat treatmentstep to obtain the perovskite-type composite oxide.

The heat treatment is not specifically limited and may be carried out,for example, by baking the precursor of the perovskite-type compositeoxide at 500° C. to 1000° C. and preferably at 500° C. to 850° C. underan oxidative atmosphere.

When the perovskite-type composite oxide is produced in theabove-mentioned manner, the heat treatment is free from an abruptexothermic reaction, is substantially free from bubbling over of theresulting powder and can be industrially efficiently carried out.

The heat treatment by-produces organic substances from the decomposedorganometal salts but does not by-produce harmful by-products. Thus, thesafety and hygiene can be improved.

In addition, by producing the perovskite-type composite oxide in theabove-mentioned manner, the perovskite-type composite oxide of a singlephase can be formed even at relatively low temperatures. This preventsthe specific surface area of the resulting perovskite-type compositeoxide from decreasing.

The perovskite-type composite oxide prepared according to the method cancomprise the noble metal efficiently dispersed therein and can have anincreased rate of solid-solution. In the perovskite-type composite oxideproduced by the method, the noble metal is finely and highly dispersedthereby to maintain its high catalytic activity even in long-term use.This is because of the self-regenerative function in which the noblemetal repetitively undergoes solid-solution under an oxidativeatmosphere and deposition under a reducing atmosphere.

Thus, the perovskite-type composite oxide produced by the method canmaintain the catalytic activity of the noble metal at a high level overa long time and can be advantageously used as an exhaust gas purifyingcatalyst, particularly as an exhaust gas purifying catalyst forautomobiles.

When the perovskite-type composite oxide produced by the method is usedas an exhaust gas purifying catalyst, the produced perovskite-typecomposite oxide may be used intact or may be processed into an exhaustgas purifying catalyst according to a conventional procedure such assupporting on a catalyst carrier.

The catalyst carrier can be any of known catalyst carriers such ashoneycomb monolith carriers derived from cordierite, without beinglimited to a particular catalyst carrier.

The produced perovskite-type composite oxide is supported by thecatalyst carrier, for example, by adding water thereto to obtain aslurry, applying the slurry to the catalyst carrier, drying the appliedslurry and subjecting the same to heat treatment at about 300° C. to800° C., and preferably at about 300° C. to 600° C.

EXAMPLES

The present invention will be illustrated in further detail withreference to several examples and comparative examples below, which arenever intended to limit the scope of the present invention.

Example 1

Initially, an alkoxide mixed solution was prepared by charging 36.6 g(0.090 mol) of lanthanum methoxypropylate [La(OCHMeCH₂OMe)₃], 4.1 g(0.010 mol) of cerium methoxypropylate [Ce(OCHMeCH₂OMe)₃], 18.4 g (0.057mol) of iron methoxypropylate [Fe(OCHMeCH₂OMe)₃], and 9.0 g (0.038 mol)of cobalt methoxypropylate [Co(OCHMeCH₂OMe)₂] in a 1000-mLround-bottomed flask and dissolving them in 200 mL of toluene addedthereto with stirring.

Separately, 1.52 g (0.005 mol) of palladium acetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaCeFeCoPd.

Next, 200 mL of deionized water was added dropwise to the round-bottomedflask over about fifteen minutes to form a viscous brown precipitate onhydrolysis.

After stirring at room temperature for two hours, toluene and water weredistilled off under reduced pressure to obtain a precursor of theLaCeFeCoPd composite oxide. The precursor was placed on a petri dish,subjected to forced-air drying at 60° C. for twenty-four hours,subjected to heat treatment at 600° C. in the atmosphere for two hoursusing an electric furnace to obtain a blackish brown powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.90)Ce_(0.10)Fe_(0.57)Co_(0.38)Pd_(0.05)O₃. The powder was found tohave a specific surface area of 36 m²/g and a Pd content in thecomposite oxide of 2.16% by mass.

Example 2

Initially, an alkoxide mixed solution was prepared by charging 40.6 g(0.100 mol) of lanthanum methoxypropylate [La(OCHMeCH₂OMe)₃], 18.4 g(0.057 mol) of iron methoxypropylate [Fe(OCHMeCH₂OMe)₃], and 8.9 g(0.038 mol) of manganese methoxypropylate [Mn(OCHMeCH₂OMe)₂] in a1000-mL round-bottomed flask and dissolving them in 200 mL of tolueneadded thereto with stirring.

Separately, 1.52 g (0.005 mol) of palladium acetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 100 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaFeMnPd.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(1.00)FE_(0.57)Mn_(0.38)Pd_(0.05)O₃. Thepowder was found to have a specific surface area of 36 m²/g, and, in thecomposite oxide, a Pd content of 2.17% by mass.

Example 3

Initially, an alkoxide mixed solution was prepared by charging 32.5 g(0.080 mol) of lanthanum ethoxyethylate [La(OC₂H₄OEt)₃], 8.2 g (0.020mol) of neodymium ethoxyethylate [Nd(OC₂H₄OEt)₃], and 29.1 g (0.090 mol)of iron ethoxyethylate [Fe(OC₂H₄OEt)₃] in a 1000-mL round-bottomed flaskand dissolving them in 200 mL of xylene added thereto with stirring.

Separately, 3.05 g (0.010 mol) of palladium cetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of xylene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaNdFePd.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(0.80)Nd_(0.90)Fe_(0.90)Pd_(0.10)O₃. Thepowder was found to have a specific surface area of 28 m²/g, and, in thecomposite oxide, a Pd content of 4.28% by mass.

Example 4

Initially, an alkoxide mixed solution was prepared by charging 25.5 g(0.070 mol) of lanthanum methoxyethylate [La(OC₂H₄OMe)₃], 7.3 g (0.020mol) of praseodymium methoxyethylate [Pr(OC₂H₄OMe)₃], 1.9 g (0.010 mol)of calcium methoxyethylate [Ca(OC₂H₄OMe)₂], 19.7 g (0.070 mol) of ironmethoxyethylate [Fe(OC₂H₄OMe)₃], and 5.1 g (0.025 mol) of manganesemethoxyethylate [Mn(OC₂H₄OMe)₂] in a 1000-mL round-bottomed flask anddissolving them in 200 mL of benzene added thereto with stirring.

Separately, 1.52 g (0.005 mol) of palladium acetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of benzene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaPrCaFeMnPd.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.70)Pr_(0.20)Ca_(0.10)Fe_(0.70)Mn_(0.25)Pd_(0.05)O₃. The powder wasfound to have a specific surface area of 30 m²/g, and, in the compositeoxide, a Pd content of 2.26% by mass.

Example 5

Initially, an alkoxide mixed solution was prepared by charging 43.6 g(0.100 mol) of lanthanum methoxypropylate [La(OCHMeCH₂OMe)₃] and 33.5 g(0.095 mol) of iron methoxypropylate [Fe(OCHMeCH₂OMe)₃] in a 1000-mLround-bottomed flask and dissolving them in 200 mL of toluene addedthereto with stirring.

Separately, 1.52 g (0.005 mol) of palladium acetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaFePd.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(1.00)Fe_(0.95)Pd_(0.05)O₃. The powder wasfound to have a specific surface area of 28 m²/g, and, in the compositeoxide, a Pd content of 2.17% by mass.

Example 6

Initially, an alkoxide mixed solution was prepared by charging 32.5 g(0.080 mol) of lanthanum methoxypropylate [La(OCHMeCH₂OMe)₃], 6.2 g(0.015 mol) of neodymium methoxypropylate [Nd(OCHMeCH₂OMe)₃], 2.0 g(0.005 mol) of cerium methoxypropylate [Ce(OCHMeCH₂OMe)₃], 24.2 g (0.075mol) of iron methoxypropylate [Fe(OCHMeCH₂OMe)₃], and 4.7 g (0.020 mol)of nickel methoxypropylate [Ni(OCHMeCH₂OMe)₂] in a 1000-mLround-bottomed flask and dissolving them in 200 mL of toluene addedthereto with stirring.

Separately, 2.00 g (0.005 mol) of rhodium acetylacetonate[Rh(CH₃COCHCOCH₃)₃] was dissolved in 200 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaNdCeFeNiRh.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.80)Nd_(0.15)Ce_(0.05)Fe_(0.75)Ni_(0.20)Rh_(0.05)O₃. The powder wasfound to have a specific surface area of 32 m²/g, and, in the compositeoxide, a Rh content of 2.09% by mass.

Example 7

Initially, an alkoxide mixed solution was prepared by charging 31.6 g(0.100 mol) of lanthanum i-propoxide [La(O^(i−)C₃H₇)₃] and 19.4 g (0.095mol) of aluminum i-propoxide [Al(O^(i−)C₃H₇)₃] in a 1000-mLround-bottomed flask and dissolving them in 200 mL of benzene addedthereto with stirring.

Separately, 2.00 g (0.005 mol) of rhodium acetylacetonate[Rh(CH₃COCHCOCH₃)₃] was dissolved in 200 mL of benzene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaAlRh.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 800° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(1.00)Al_(0.95)Rh_(0.05)O₃. The powder wasfound to have a specific surface area of 24 m²/g, and, in the compositeoxide, a Rh content of 2.34% by mass.

Next, the powder was impregnated with 25.9 g (corresponding to 0.22 g ofPt) of a dinitrodiammine platinum nitrate solution having a Pt contentof 8.50% by mass, subjected to forced-air drying at 60° C. fortwenty-four hours, subjected to heat treatment at 500° C. in theatmosphere for one hour using an electric furnace to obtain aPt-supporting/La_(1.00)Al_(0.95)Rh_(0.05)O₃ powder. The amount of Ptsupported was 1.00% by mass.

Example 8

Initially, an alkoxide mixed solution was prepared by charging 32.2 g(0.090 mol) of lanthanum n-butoxide [La(O^(n−)C₄H₉)₃], 3.1 g (0.010 mol)of yttrium n-butoxide [Y(O^(n−)C₄H₉)₃], 19.3 g (0.070 mo1) of ironn-butoxide [Fe(O^(n−)C₄H₉)₃], and 4.9 g (0.020 mol) of aluminumn-butoxide [Al (O^(n−)C₄H₉)₃] in a 1000-mL round-bottomed flask anddissolving them in 200 mL of toluene added thereto with stirring.

Separately, 4.00 g (0.01 mol) of rhodium acetylacetonate[Rh(CH₃COCHCOCH₃)₃] was dissolved in 200 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaYFeAlRh.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 500° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.90)Y_(0.10)Fe_(0.70)Al_(0.20)Rh_(0.10)O₃. The powder was found tohave a specific surface area of 21 m²/g, and, in the composite oxide, aRh content of 4.35% by mass.

Example 9

Initially, an alkoxide mixed solution was prepared by charging 37.0 g(0.090 mol) of neodymium ethoxyethylate [Nd(OC₂H₄OEt)₃], 3.2 g (0.010mol) of barium ethoxyethylate [Ba(OC₂H₄OEt)₂], 25.8 g (0.080 mol) ofiron ethoxyethylate [Fe(OC₂H₄OEt)₃], and 2.4 g (0.010 mol) of copperethoxyethylate [Cu(OC₂H₄OEt)₂] in a 1000-mL round-bottomed flask anddissolving them in 200 mL of xylene added thereto with stirring.

Separately, 4.90 g (0.010 mol) of iridium acetylacetonate[Ir(CH₃COCHCOCH₃)₃] was dissolved in 200 mL of xylene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing NdBaFeCuIr.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 500° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofNd_(0.90)Ba_(0.10)Fe_(0.80)Cu_(0.10)Ir_(0.10)O₃. The powder was found tohave a specific surface area of 18 m²/g, and, in the composite oxide, anIr content of 7.34% by mass.

Example 10

Initially, an alkoxide mixed solution was prepared by charging 36.6 g(0.090 mol) of lanthanum ethoxyethylate [La(OC₂H₄OEt)₃], 2.7 g (0.010mol) of strontium ethoxyethylate [Sr(OC₂H₄OEt)₂], 17.4 g (0.054 mol) ofiron ethoxyethylate [Fe(OC₂H₄OEt)₃], and 8.5 g (0.036 mol) of cobaltethoxyethylate [Co(OC₂H₄OEt)₂] in a 1000-mL round-bottomed flask anddissolving them in 200 mL of toluene added thereto with stirring.

Separately, 3.93 g (0.010 mol) of platinum acetylacetonate[Pt(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaSrFeCoPt.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.90)Sr_(0.10)Fe_(0.54)Co_(0.36)Pt_(0.10)O₃. The powder was found tohave a specific surface area of 28 m²/g, and, in the composite oxide, aPt content of 7.72% by mass.

Example 11

Initially, an alkoxide mixed solution was prepared by charging 34.6 g(0.095 mol) of lanthanum methoxyethylate [La(OC₂H₄OMe)₃], 20.2 g (0.080mol) of aluminum methoxyethylate [Al(OC₂H₄OMe)₃], and 2.0 g (0.010 mol)of manganese methoxyethylate [Mn(OC₂H₄OMe)₂] in a 1000-mL round-bottomedflask and dissolving them in 200 mL of toluene added thereto withstirring.

Separately, 1.04 g (0.005 mol) of silver acetylacetonate[Ag(CH₃COCHCOCH₃)], 3.14 g (0.008 mol) of platinum acetylacetonate[Pt(CH₃COCHCOCH₃)₂], and 0.80 g (0.002 mol) of ruthenium acetylacetonate[Ru(CH₃COCHCOCH₃)₃] were dissolved in 200 mL of toluene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing LaAgAlMnPtRu.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 800° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.95)Ag_(0.05)Al_(0.80)Mn_(0.10)Pt_(0.08)Ru_(0.02)O₃. The powder wasfound to have a specific surface area of 19 m²/g, and, in the compositeoxide, a Ag content of 2.42% by mass, a Pt content of 6.78% by mass, anda Ru content of 0.88% by mass.

Example 12

Initially, an alkoxide mixed solution was prepared by charging 32.9 g(0.080 mol) of neodymium methoxypropylate [Nd(OCHMeCH₂OMe)₃], 3.2 g(0.010 mol) of barium methoxypropylate [Ba(OCHMeCH₂OMe)₂], 2.0 g (0.010mol) of magnesium methoxypropylate [Mg(OCHMeCH₂OMe)₂], and 25.0 g (0.085mol) of aluminum methoxypropylate [Al(OCHMeCH₂OMe)₃] in a 1000-mLround-bottomed flask and dissolving them in 200 mL of xylene addedthereto with stirring.

Separately, 3.93 g (0.010 mol) of platinum acetylacetonate[Pt(CH₃COCHCOCH₃)₂] and 2.00 g (0.005 mol) of rhodium acetylacetonate[Rh(CH₃COCHCOCH₃)₃] were dissolved in 200 mL of xylene to obtain anorganometal salt solution, and the organometal salt solution was furtheradded to the alkoxide mixed solution in the round-bottomed flask toobtain a homogenous mixed solution containing NdBaMgAlPtRh.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofNd_(0.80)Ba_(0.10)Mg_(0.10)Al_(0.85)Pt_(0.10)Rh_(0.05)O₃. The powder wasfound to have a specific surface area of 29 m²/g, and, in the compositeoxide, a Pt content of 8.59% by mass and a Rh content of 2.27% by mass.

Example 13

Initially, an organometal salt solution was prepared by charging 43.6 g(0.100 mol) of lanthanum acetylacetonate [La(CH₃COCHCOCH₃)₃], 21.2 g(0.060 mol) of iron acetylacetonate [Fe(CH₃COCHCOCH₃)₃], and 12.2 g(0.040 mol) of palladium acetylacetonate [Pd(CH₃COCHCOCH₃)₂] in a1000-mL round-bottomed flask and dissolving them in 300 mL of tolueneadded thereto with stirring.

Next, toluene was distilled off under reduced pressure to obtain amixture. The mixture was thermally decomposed by heating to 400° C. inthe atmosphere using an electric furnace to obtain a precursor of aLaFePd composite oxide. Then, the precursor was subjected to the heattreatment at 700° C. in the atmosphere for two hours using an electricfurnace to obtain a blackish brown powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(1.00)Fe_(0.60)Pd_(0.40)O₃. The powder wasfound to have a specific surface area of 24 m²/g, and, in the compositeoxide, a Pd content of 16.2% by mass.

Example 14

Initially, an aqueous mixed salt solution was prepared by charging 43.3g (0.100 mol) of lanthanum nitrate hexahydrate (La(NO₃)₃.6H₂O) and 36.4g (0.090 mol) of iron nitrate enneahydrate (Fe(NO₃)₃.9H₂O) in a 1000-mLround-bottomed flask and dissolving and homogeneously mixing in 100 mLof deionized water. Next, 47.9 g (0.23 mol) of citric acid was dissolvedin 100 mL of deionized water, and the resulting solution was added tothe aqueous mixed salt solution to obtain an aqueous solution of citricacid and salts containing LaFe.

The aqueous solution of citric acid and salts was evaporated to drynesson a hot water bath at 60° C. to 80° C. with evacuation using a rotaryevaporator. After passage of about three hours and when the solutioncame into a starch-syrup-like state, the temperature of the hot waterbath was gradually raised, followed by drying at 300° C. in vacuum forone hour to obtain a citrate complex.

The above-prepared citrate complex was pulverized in

a mortar and baked at 350° C. in the atmosphere for three hours and thencharged again in the 1000-mL flask.

Then, 3.05 g (0.010 mol) of palladium acetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 200 mL of acetone, and theresulting solution was charged in the round-bottomed flask, followed bystirring to obtain a homogeneous slurry containing LaFePd.

Acetone in the homogenous slurry was evaporated to dryness and theresulting powder was subjected to the heat treatment at 700° C. in theatmosphere for three hours using an electric furnace to obtain ablackish brown powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(1.00)Fe_(0.90)Pd_(0.10)O₃. The powder wasfound to have a specific surface area of 28 m²/g and a Pd content in thecomposite oxide of 4.31% by mass.

Example 15

Initially, an aqueous mixed salt solution was prepared by charging 37.1g (0.100 mol) of lanthanum chloride heptahydrate [LaCl₃.7H₂O] and 21.6 g(0.080 mol) of iron chloride hexahydrate [FeCl₃.6H₂O] in a 1000-mLround-bottomed flask and dissolving and homogeneously mixing in 200 mLof deionized water. Next, the aqueous mixed salt solution was graduallyadded dropwise in an aqueous alkali solution prepared by 208 g ofammonium carbonate having a NH₃ content of 30% by weight in 200 mL ofdeionized water to obtain a coprecipitate. After stirring at roomtemperature for two hours, the resulting coprecipitate was sufficientlywashed with water and then filtered.

The coprecipitate was placed on a petri dish and sufficiently dried byforced-air drying at 80° C. for twelve hours, pulverized in a mortar andthen charged again in the 1000-mL flask.

Then, 6.09 g (0.020 mol) of palladium acetylacetonate[Pd(CH₃COCHCOCH₃)₂] was dissolved in 400 mL of acetone, and theresulting solution was charged in the round-bottomed flask, followed bystirring to obtain a homogeneous slurry containing LaFePd.

Acetone in the homogenous slurry was evaporated to dryness and theresulting powder was subjected to the heat treatment at 700° C. in theatmosphere for three hours using an electric furnace to obtain ablackish brown powder.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure of La_(1.00)Fe_(0.80)Pd_(0.20)O₃. The powder wasfound to have a specific surface area of 32 m²/g and a Pd content in thecomposite oxide of 8.31% by mass.

Comparative Example 1

Initially, an alkoxide mixed solution was prepared by charging 36.6 g(0.090 mol) of lanthanum methoxypropylate [La(OCHMeCH₂OMe)₃], 4.1 g(0.010 mol) of cerium methoxypropylate [Ce(OCHMeCH₂OMe)₃], 18.4 g (0.057mol) of iron methoxypropylate [Fe(OCHMeCH₂OMe)₃], and 9.0 g (0.038 mol)of cobalt methoxypropylate [Co(OCHMeCH₂OMe)₂] in a 1000-mLround-bottomed flask and dissolving them in 200 mL of toluene addedthereto with stirring.

Next, an aqueous solution prepared by diluting 12.0 g (corresponding to0.53 g (0.005 mol) of Pd) of an aqueous palladium nitrate solutionhaving a Pd content of 4.4% by mass with 100 mL of deionized water wasadded dropwise to the round-bottomed flask over about fifteen minutes toform a viscous brown precipitate on hydrolysis.

Subsequently, a blackish brown powder was obtained by the procedure ofExample 1, except for carrying out the heat treatment at 850° C. for twohours.

The X-ray powder diffraction analysis of the powder revealed that it wasidentified as a single crystal phase comprising a composite oxide havinga perovskite structure ofLa_(0.90)Ce_(0.10)Fe_(0.57)Co_(0.38)Pd_(0.05)O₃. The powder was found tohave a specific surface area of 30 m²/g, and, in the composite oxide, aPd content of 2.16% by mass.

Comparative Example 2

A total of 100 g of a commercially available γ-Al₂O₃ having a specificsurface area of 180 m²/g was impregnated with Pd using 50.2 g(corresponding to 2.21 g of Pd) of an aqueous palladium nitrate solutionhaving a Pd content of 4.4% by mass, subjected to forced-air drying at60° C. for twenty-four hours and then subjected to the heat treatment at500° C. in the atmosphere for one hour using an electric furnace. Theamount of Pd supported by γ-Al₂O₃ was 2.16% by mass.

Comparative Example 3

A total of 20 g of a commercially available γ-Al₂O₃ having a specificsurface area of 180 m²/g was impregnated with Rh using 9.6 g(corresponding to 0.43 g of Rh) of an aqueous rhodium nitrate solutionhaving a Rh content of 4.48% by mass, subjected to forced-air drying at60° C. for twenty-four hours and then subjected to the heat treatment at500° C. in the atmosphere for one hour using an electric furnace. Theamount of Rh supported by γ-Al₂O₃ was 2.10% by mass.

Comparative Example 4

A total of 20 g of a commercially available γ-Al₂O₃ having a specificsurface area of 180 m²/g was impregnated with Pd using 10.5 g(corresponding to 1.03 g of Pd) of an aqueous palladium nitrate solutionhaving a Pd content of 9.83% by mass, subjected to forced-air drying at60° C. for twenty-four hours and then subjected to the heat treatment at500° C. in the atmosphere for one hour using an electric furnace. Theamount of Pd supported by γ-Al₂O₃ was 4.90% by mass.

Comparative Example 5

A total of 20 g of a commercially available γ-Al₂O₃ having a specificsurface area of 180 m²/g was impregnated with Pt using 27.1 g(corresponding to 2.3 g of Pt) of a dinitrodiammine platinum nitratesolution having a Pt content of 8.50% by mass, subjected to forced-airdrying at 60° C. for twenty-four hours and then subjected to heattreatment at 500° C. in the atmosphere for one hour using an electricfurnace. The amount of Pt supported by γ-Al₂O₃ was 10.3% by mass.

Test Example 1

1) Coating to Catalyst Carrier

A total of 100 mL of deionized water was mixed with 100 g of the powdersprepared according to Example 1 and Comparative Examples 1 and 2,followed by addition of 17.5 g of zirconia sol (NZS-30B, a product ofNissan Chemical Industries, Ltd.; a solid content of 30% by mass) toobtain a slurry. The slurry was applied by coating to a catalyst carriercomprising a cordierite honeycomb having a diameter of 80 mm, a lengthof 95 mm, and a grating density of 400 cells/(0.025 m)².

After coating, excess slurry was removed by air blow so as to set thecoating amount of the powder at 157.5 g per 1 L of the catalyst carrier(75.1 g per one catalyst carrier). After forced-air drying at 120° C.for twelve hours, the work was baked at 600° C. in the atmosphere forthree hours to obtain monolith catalysts containing the powdersaccording to Example 1 and Comparative Examples 1 and 2, respectively.

2) Endurance Test

The above-prepared monolith catalysts were connected to both banks of aV type eight cylinder engine having a displacement of 4 liters and weresubjected to an endurance test at a highest temperature in the catalystbed of 1050° C. with a single cycle of 30 seconds repeated for a totalof 40 hours.

One cycle of the endurance test was set as follows. Specifically, fromSecond 0 to Second 5 (a period of 5 seconds), the operation was carriedout at a theoretical fuel-air ratio (λ=1). From Second 5 to Second 28 (aperiod of 23 seconds), an excessive amount of fuel was fed to the bed(λ=0.89). From Second 7 to Second 30 (a period of 23 seconds) laggingtwo seconds from the above, high-pressure secondary air was introducedupstream of the catalysts. From Second 7 to Second 28 (a period of 21seconds), a slightly excessive amount of air was fed (λ=1.02) to causethe excessive fuel to burn in the interior of the bed, so as to raisethe temperature of the catalyst bed to 1050° C. From Second 28 to Second30 (a period of 2 seconds), the interior of the bed was returned to thetheoretical fuel-air ratio (λ=1) and the secondary air was kept to befed to achieve a high-temperature oxidative atmosphere in which the airis in large excess (λ=1.25).

3) Activity Measurement

Using an in-line four-cylinder engine having a displacement of 1.5liters, an oscillation (amplitude) of Δλ=±3.4% (ΔA/F=±0.5 A/F) of whichthe center was set in the theoretical fuel-air ratio (λ=1) was appliedto the monolith catalysts at a frequency of 1 Hz. The purification ratesof CO, HC, and NOx of the monolith catalysts before and after thisendurance test were measured. The results are shown in Table 1. In themeasurement, the temperature of the upstream (inlet gas) of the monolithcatalysts was kept at 460° C. and the flow rate was set at a spacevelocity (SV) of 50000 per hour. Table 1 also shows the Pd content (g)per 1 liter of each of the monolith catalysts. TABLE 1 Purification ratePurification rate before endurance after endurance Pd content test (%)test (%) Catalyst Composition (g/L catalyst) CO HC NOx CO HC NOx Example1 La_(0.90)Ce_(0.10)Fe_(0.57)Co_(0.38)Pd_(0.05)O₃ 3.24 98.1 97.8 99.790.6 89.0 91.5 ComparativeLa_(0.90)Ce_(0.10)Fe_(0.57)Co_(0.38)Pd_(0.05)O₃ 3.24 98.5 97.6 99.8 84.982.1 84.1 Example 1 Comparative Pd-supporting/γ-Al₂O₃ 3.24 97.5 99.399.3 48.3 75.1 78.2 Example 2

Table 1 shows that the monolith catalyst comprising the powder accordingto Comparative Example 2 exhibits markedly decreased purification ratesafter the endurance test, and that, in contrast, the monolith catalystcomprising the powder according to Example 1 maintains their highactivities even after the endurance test.

Table 1 also shows that, although the catalysts according to Example 1and Comparative Example 1 have the same composition, the monolithcatalyst comprising the powder according to Example 1 produced by themethod of the present invention causes less deterioration after theendurance test as compared with the monolith catalyst comprising thepowder according to Comparative Example 1.

Test Example 2

1) Coating to Catalyst Carrier

A total of 120 mL of deionized water was mixed with 20 g of the powdersprepared according to Examples 2, 4, 6, and 7 and Comparative Examples 2and 3 and 100 g of a powdery composite oxide having a composition ofCe_(0.6)Zr_(0.3)Y_(0.1)O_(0.95), followed by addition of 21.1 g ofzirconia sol (NZS-30B, a product of Nissan Chemical Industries, Ltd.; asolid content of 30% by mass) to obtain a slurry. The slurry was appliedby coating to a catalyst carrier comprising a cordierite honeycombhaving a diameter of 80 mm, a length of 95 mm, and a grating density of400 cells/(0.025 m)².

After coating, excess slurry was removed by air blow so as to set thecoating amount of the powder at 126 g per 1 L of the catalyst carrier(60 g per one catalyst carrier). After forced-air drying at 120° C. fortwelve hours, the work was baked at 600° C. in the atmosphere for threehours to obtain monolith catalysts containing the powders according toExamples 2, 4, 6, and 7 and Comparative Examples 2 and 3, respectively.

2) Endurance Test

The above-prepared monolith catalysts were connected to both banks of aV type eight cylinder engine having a displacement of 4 liters and weresubjected to an endurance test at a temperature in the catalyst bed of900° C. with a single cycle of 900 seconds repeated for a total of 100hours.

One cycle of the endurance test was set as follows. Specifically, fromSecond 0 to Second 870 (a period of 870 seconds), an oscillation(amplitude) of Δλ=±4% (ΔA/F=±0.6 A/F) with the theoretical fuel-airratio (λ=1) of A/F=14.6 (A/F=air to fuel ratio) at the center wasapplied to the monolith catalysts at a frequency of 0.6 Hz. From Second870 to Second 900 (a period of 30 seconds), secondary air was introducedupstream of the catalysts to achieve forced oxidation under theconditions (λ=1.25).

3) Activity Measurement

Under the same conditions as in the item 3) (activity measurement) ofTest Example 1, the purification rates of CO, HC, and NOx of themonolith catalysts before and after this endurance test were determined.The results are shown in Table 2. In the measurement, the flow rate wasset at a space velocity (SV) of 70000 per hour. Table 2 also shows thenoble metal content (g) per 1 liter of each of the monolith catalysts.TABLE 2 Purification rate Purification rate Noble metal before enduranceafter endurance content test (%) test (%) Catalyst Composition (g/Lcatalyst) CO HC NOx CO HC NOx Example 2La_(1.00)Fe_(0.57)Mn_(0.38)Pd_(0.05)O₃ Pd: 0.43 96.7 98.4 96.1 87.1 89.687.4 Example 4 La_(0.70)Pr_(0.20)Ca_(0.10)Fe_(0.70)Mn_(0.25)Pd_(0.05)O₃Pd: 0.45 95.4 95.6 94.3 86.7 87.5 86.3 Example 6La_(0.80)Nd_(0.15)Ce_(0.05)Fe_(0.75)Ni_(0.20)Rh_(0.05)O₃ Rh: 0.42 94.494.9 98.5 85.0 86.2 88.4 Example 7Pd-supporting/La_(1.00)Al_(0.95)Rh_(0.05)O₃ Rh: 0.47 97.3 98.0 99.0 88.289.3 90.8 Pt: 0.20 Comparative Pd-supporting/γ-Al₂O₃ Pd: 0.43 97.7 10098.8 52.3 73.8 67.5 Example 2 Comparative Rh-supporting/γ-Al₂O₃ Rh: 0.4298.0 98.7 99.8 79.2 72.1 83.1 Example 3

Table 2 shows that the monolith catalysts comprising the powdersaccording to Comparative Examples 2 and 3 exhibit markedly decreasedpurification rates after the endurance test, and that, in contrast, themonolith catalysts comprising the powders according to Example 2, 4, 6,and 7 maintain their high activities even after the endurance test.

Test Example 3

1) Coating to Catalyst Carrier

Using the powders obtained in Examples 3, 5, and 8 to 15 and ComparativeExamples 4 and 5, monolith catalysts containing the powders according toExamples 3, 5, and 8 to 15 and Comparative Examples 4 and 5 wereobtained, respectively, by the same procedure as in the item 1) (coatingto the catalyst carrier) of Test Example 2.

2) Endurance Test

Under the same conditions as in the item 2) (endurance test) of TestExample 1, the above-prepared monolith catalysts were connected to bothbanks of a V type eight cylinder engine having a displacement of 4liters and were subjected to an endurance test at a highest temperaturein the catalyst bed of 1050° C. with a single cycle of 30 secondsrepeated for a total of 60 hours.

3) Activity Measurement

Under the same conditions for activity measurement as in the item 3) ofTest Example 1, the purification rates of CO, HC, and NOx of themonolith catalysts before and after this endurance test were measured.The results are shown in Table 3. Table 3 also shows the noble metalcontent (g) per 1 liter of each of the monolith catalysts. TABLE 3Purification rate Purification rate Noble metal before endurance afterendurance content test (%) test (%) Catalyst Composition (g/L catalyst)CO HC NOx CO HC NOx Example 3 La_(0.80)Nd_(0.20)Fe_(0.90)Pd_(0.10)O₃ Pd:0.86 98.2 99.8 97.8 87.4 89.1 88.5 Example 5La_(1.00)Fe_(0.95)Pd_(0.05)O₃ Pd: 0.43 98.1 99.7 97.0 85.3 88.9 87.2Example 8 La_(0.90)Y_(0.10)Fe_(0.70)Al_(0.20)Rh_(0.10)O₃ Rh: 0.87 95.597.6 99.0 86.8 86.1 92.5 Example 9Nd_(0.90)Ba_(0.10)Fe_(0.80)Cu_(0.10)Ir_(0.10)O₃ Ir: 1.47 92.1 93.5 98.577.0 86.0 86.0 Example 10La_(0.90)Sr_(0.10)Fe_(0.54)Co_(0.36)Pt_(0.10)O₃ Pt: 1.54 94.3 96.8 93.885.7 88.5 85.1 Example 11La_(0.95)Ag_(0.05)Al_(0.80)Mn_(0.10)Pt_(0.08)Ru_(0.02)O₃ Ag: 0.48 96.693.4 97.2 87.6 82.0 81.7 Pt: 1.36 Ru: 0.18 Example 12Nd_(0.80)Ba_(0.10)Mg_(0.10)Al_(0.85)Pt_(0.10)Rh_(0.05)O₃ Pt: 1.72 99.3100 100 92.0 89.7 92.5 Rh: 0.45 Example 13 La_(1.00)Fe_(0.60)Pd_(0.40)O₃Pd: 3.24 99.8 100 99.8 90.5 91.6 91.4 Example 14La_(1.00)Fe_(0.90)Pd_(0.10)O₃ Pd: 0.86 99.5 99.8 99.0 86.7 90.2 88.9Example 15 La_(1.00)Fe_(0.80)Pd_(0.20)O₃ Pd: 1.72 99.6 100 99.5 88.890.8 91.1 Comparative Pd-supporting/γ-Al₂O₃ Pd: 0.98 98.2 100 100 26.157.1 50.7 Example 4 Comparative Pt-supporting/γ-Al₂O₃ Pt: 2.06 98.8 92.187.0 66.7 60.8 27.1 Example 5

Table 3 shows that the monolith catalysts comprising the powdersaccording to Comparative Examples 4 and 5 exhibit markedly decreasedpurification rates after the endurance test, and that, in contrast, themonolith catalysts comprising the powders according to Example 3, 5, and8 to 15 maintain their high activities even after the endurance test.

While the illustrative embodiments and examples of the present inventionare provided in the above description, such is for illustrative purposeonly and it is not to be construed restrictively. Modification andvariation of the present invention which will be obvious to thoseskilled in the art is to be covered in the following claims.

INDUSTRIAL APPLICABILITY

According to the method for producing a perovskite-type composite oxideof the present invention, there can be produced a perovskite-typecomposite oxide which can maintain the catalytic activity of a noblemetal at a high level over a long time. Such a perovskite-type compositeoxide is advantageously usable as an exhaust gas purifying catalyst,particularly as an automobile exhaust gas purifying catalyst.

1. A method for producing a perovskite-type composite oxide, whichcomprises the steps of: preparing a precursor of the perovskite-typecomposite oxide by mixing at least organometal salts of elementarycomponents constituting the perovskite-type composite oxide, andheat-treating the precursor of the perovskite-type composite oxide. 2.The method for producing a perovskite-type composite oxide according toclaim 1, wherein, in the preparation step, the precursor of theperovskite-type composite oxide is prepared by mixing one or moreorganometal salts of part of the elementary components constituting theperovskite-type composite oxide with the other elementary components. 3.The method for producing a perovskite-type composite oxide according toclaim 2, wherein the other elementary components is prepared asalkoxides of the respective elements.
 4. The method for producing aperovskite-type composite oxide according to claim 2, wherein the otherelementary components is prepared as a coprecipitate of salts of therespective elements or a citrate complex of the respective elements. 5.The method for producing a perovskite-type composite oxide according toclaim 2, wherein the part of the elementary components is one or morenoble metals.
 6. The method for producing a perovskite-type compositeoxide according to claim 1, wherein the organometal salts of theelementary components are organic carboxylic acid salts of theelementary components and/or a metal complex of the elementarycomponents including at least one selected from the group consisting ofβ-diketone compounds, β-ketoester compounds and β-dicarboxylic estercompounds.
 7. The method for producing a perovskite-type composite oxideaccording to claim 1, wherein the perovskite-type composite oxide is aperovskite-type composite oxide represented by the following generalformula (1):ABMO₃  (1) wherein A represents at least one element selected fromrare-earth elements, alkaline earth metals, and Ag; B represents atleast one element selected from Al and transition metals excludingplatinum group elements and rare-earth elements; and M represents one ormore platinum group elements.